Seismic velocities and depth conversion: Cenozoic (excl. Danian) of the Danish Central Trough, North Sea

Travel times and depths for a number of lithostratigraphic units encountered in 59 wells have been studied to investigate the regional variations in the seismic velocity of the Cenozoic (excl. Danian) of the Danish Central Trough, North Sea. Provisional results from this study are presented. The investigation was ai med at optimizing depth conversion down to top of the Upper Cretaceous-Danian Chalk Group, an important hydrocarbon reservoir. Whereas the thickness of the Cenozoic (excl. Danian) (or the Post Chalk Group) varies from 1725 to 3060 m, the interval velocities measured over the whole unit range only from 2006 to 2114 m/sec. This variation in velocity may cause depth prognosis errors of up to 100 m in the area. A significant velocity inversion at top Middle Miocene separates the sequence into two thick units with different velocity characteristics. The main velocity variations of the Cenozoic (excl. Danian) are caused by gross variations in the lower unit, probably related to lateral differences in pressure and lithology. The upper unit, which is dominated by shales and sands, is normally compacted, wheras the lower unit, which is dominated by shales, is undercompacted due to overpressure. By estimating the drop in velocity at the interface between the two units, a measure of the overpressure below the surface is derived. It is found that the overpressure increases with the thickness of the overburden. The interval velocities of the thick deposits of the lower unit in the NW area are comparable to the velocities of the thinner deposits in the SE. The reason for this is suggested to be the higher overpressure in the NW.